But they need bulk optical parts such as birefringence plates
and special launching lenses and thus precision alignment and
careful handling are needed [1], [5]–[10]. In this scenario,
isolators made by using optical fibers with large Faraday effect
are of great interest for all-optical device applications, fiber
laser system and fiber optic amplifiers because they offer
advantages like low insertion loss, high return loss, and high
isolation together with no need of bulky optical parts and
precise alignment [11]–[15].
Recently, a diode laser or a second-harmonic signal source
operating at 660 nm is known to be used as a laser source in
fiber laser systems for industrial and bio-medical applications
[16]–[18]. However, in our best knowledge, an all-optical fiber
isolator operating at 660 nm wavelength has been not yet reported;
mainly due to the low sensitivity of silica glass fiber,
which is attributed to its low magneto-optic sensitivity at visible
wavelength (Verdet constant at 1550 nm and
at 1310 nm [19]–[21]). Even though specialty
optical fibers such as annealed fiber, twisted fiber, spun fiber
and flint glass fiber were suggested to improve the magnetooptic
sensitivity by reducing linear birefringence of fibers [22],
[23], drawbacks such as complicated fabrication process, high
splicing loss, and high cost are still bottlenecks in their mass
production. To overcome these problems, phosphate or borosilicate
glass optical fibers incorporated with Tb-ions having high
Verdet constant were reported where a rod-in-tube technique
was used [11]; the propagation loss of fiber was found to be
significantly higher than that of silica glass fiber. In the current
communication, we report the development and demonstration
of the all-optical fiber isolator based on the specialty optical
fiber doped with CdSe quantum dots (QDs) for enhanced
magneto-optic sensitivity, which is the first reported isolator allowing
optical isolation at 660 nm [24].